U.S. patent application number 15/042877 was filed with the patent office on 2016-09-29 for catalytic converter.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Mitsuyoshi OKADA, Yoshinori SAITO, Hiromasa SUZUKI, Takeru YOSHIDA.
Application Number | 20160279613 15/042877 |
Document ID | / |
Family ID | 56890301 |
Filed Date | 2016-09-29 |
United States Patent
Application |
20160279613 |
Kind Code |
A1 |
SUZUKI; Hiromasa ; et
al. |
September 29, 2016 |
CATALYTIC CONVERTER
Abstract
A catalytic converter with excellent OSC performance and
NO.sub.x purification performance. The converter includes a
substrate with a cell structure through which exhaust gas flows,
and a catalyst layer formed on a cell wall surface of the
substrate. The catalyst layer includes a lower catalyst layer and
an upper catalyst layer, the lower catalyst layer being formed on a
surface of the substrate, and the upper catalyst layer being formed
on a surface of the lower catalyst layer. The upper catalyst layer
includes at least a zirconia support with rhodium carried thereon,
and two types of ceria-zirconia-based composite oxides with
different specific surface areas, each of the ceria-zirconia-based
composite oxides having no rhodium carried thereon. The lower
catalyst layer includes an alumina support with platinum carried
thereon, and a ceria-zirconia-based composite oxide.
Inventors: |
SUZUKI; Hiromasa;
(Toyota-shi, JP) ; YOSHIDA; Takeru; (Miyoshi-shi,
JP) ; SAITO; Yoshinori; (Toyota-shi, JP) ;
OKADA; Mitsuyoshi; (Toyota-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TOYOTA JIDOSHA KABUSHIKI KAISHA |
Toyota-shi |
|
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
|
Family ID: |
56890301 |
Appl. No.: |
15/042877 |
Filed: |
February 12, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01J 35/0006 20130101;
B01D 2255/2092 20130101; B01D 2255/1025 20130101; B01J 35/04
20130101; B01J 35/1009 20130101; B01J 2523/36 20130101; B01J
2523/3706 20130101; B01J 2523/828 20130101; B01J 2523/3725
20130101; B01J 2523/48 20130101; B01J 2523/822 20130101; B01J
2523/31 20130101; B01J 2523/48 20130101; B01J 2523/822 20130101;
B01J 2523/31 20130101; B01J 2523/36 20130101; B01J 2523/828
20130101; B01J 2523/3706 20130101; B01J 2523/3725 20130101; B01J
2523/3712 20130101; B01J 2523/3712 20130101; B01J 2523/36 20130101;
B01J 2523/3706 20130101; B01J 2523/3712 20130101; B01J 2523/48
20130101; B01D 2255/20715 20130101; B01D 2255/9022 20130101; B01J
2523/00 20130101; B01J 2523/00 20130101; B01D 2255/1021 20130101;
B01D 53/9422 20130101; B01D 2255/906 20130101; B01J 37/0248
20130101; B01J 23/63 20130101; B01D 2255/407 20130101; B01J 2523/00
20130101; B01J 35/1014 20130101; B01J 37/0244 20130101; B01J
2523/00 20130101; B01J 2523/31 20130101 |
International
Class: |
B01J 23/63 20060101
B01J023/63; B01J 35/04 20060101 B01J035/04; B01J 35/10 20060101
B01J035/10; B01J 35/00 20060101 B01J035/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 23, 2015 |
JP |
2015-059384 |
Claims
1. A catalytic converter comprising: a substrate with a cell
structure through which exhaust gas flows; and a catalyst layer
formed on a cell wall surface of the substrate, wherein the
catalyst layer includes a lower catalyst layer and an upper
catalyst layer, the lower catalyst layer being formed on a surface
of the substrate, and the upper catalyst layer being formed on a
surface of the lower catalyst layer, the upper catalyst layer
includes at least a zirconia support with rhodium carried thereon,
and two types of ceria-zirconia-based composite oxides with
different specific surface areas, each of the ceria-zirconia-based
composite oxides having no rhodium carried thereon, and the lower
catalyst layer includes an alumina support with platinum carried
thereon, and a ceria-zirconia-based composite oxide.
2. The catalytic converter according to claim 1, wherein, of the
two types of ceria-zirconia-based composite oxides with different
specific surface areas, the ceria-zirconia-based composite oxide
with a larger specific surface area has a specific surface area of
greater than or equal to 40 m2/g, and is contained in the upper
catalyst layer by an amount of greater than or equal to 12 g/L, and
the ceria-zirconia-based composite oxide with a smaller specific
surface area has a specific surface area of less than or equal to 4
m2/g, and is contained in the upper catalyst layer by an amount of
greater than or equal to 8 g/L.
3. The catalytic converter according to claim 1, wherein the
zirconia support with rhodium carried thereon is contained in the
upper catalyst layer by an amount of 40 g/L.
4. The catalytic converter according to claim 2, wherein the
zirconia support with rhodium carried thereon is contained in the
upper catalyst layer by an amount of 40 g/L.
Description
CLAIM OF PRIORITY
[0001] The present application claims priority from Japanese patent
application JP 2015-059384 filed on Mar. 23, 2015, the content of
which is hereby incorporated by reference into this
application.
BACKGROUND
[0002] 1. Technical Field
[0003] The present invention relates to a catalytic converter that
is fixedly housed in a pipe of an exhaust system for exhaust
gas.
[0004] 2. Background Art
[0005] In a variety of industries, a variety of attempts has been
made worldwide to reduce environmental impacts and burdens. In
particular, in the automobile industry, development for promoting
the spread of not only fuel-efficient gasoline engine vehicles, but
also so-called eco-friendly vehicles, such as hybrid vehicles and
electric vehicles, as well as for further improving the performance
of such vehicles has been advanced day by day. In addition to the
development of such eco-friendly vehicles, research about an
exhaust gas purifying catalyst for purifying exhaust gas discharged
from an engine has also been actively conducted. An exhaust gas
purifying catalyst contains an oxidation catalyst, a three-way
catalyst, a NO.sub.x, storage-reduction catalyst, and the like. A
noble metal catalyst, such as platinum (Pt), palladium (Pd), or
rhodium (Rh), exhibits catalytic activity in the exhaust gas
purifying catalyst. The noble metal catalyst is typically used
while being carried on a support that is made of porous oxide, such
as alumina (Al.sub.2O.sub.3).
[0006] A catalytic converter for purifying exhaust gas is typically
arranged in an exhaust system for exhaust gas that connects a
vehicle engine and a muffler. The engine may sometimes discharge
environmentally harmful substances, such as CO, NO.sub.x, and
unburned HC and VOC. In order to convert such harmful substances
into allowable substances, exhaust gas is passed through a
catalytic converter in which a catalyst layer, which has a noble
metal catalyst, such as Rh, Pd, or Pt carried on a support, is
arranged on the cell wall surface of a substrate, so that CO is
converted into CO.sub.2, and NO.sub.x is converted into N.sub.2 and
O.sub.2, while HC and VOC are burned to generate CO.sub.2 and
H.sub.2O.
[0007] As a support on which a noble metal catalyst is carried, a
ceria-zirconia-based composite oxide (which is also referred to as
a CeO.sub.2--ZrO.sub.2 solid solution, a CZ material, and the like)
can be used. This is also referred to as a promoter, and is an
essential component of the aforementioned three-way catalyst for
concurrently removing CO, NO.sub.x, and HC that are harmful
components in the exhaust gas. Examples of the essential component
of the promoter include CeO.sub.2. CeO.sub.2 has a property that
its oxidation number changes to Ce.sup.3+ or Ce.sup.4+, for
example, depending on the partial pressure of oxygen in the exhaust
gas to which CeO.sub.2 is exposed, and has a function of absorbing
and releasing oxygen as well as a function of storing oxygen (OSC:
Oxygen Storage Capacity) to compensate for the deficiency and
excess of electric charges. In addition, CeO.sub.2 can absorb and
mitigate fluctuations of the exhaust gas atmosphere and maintain
the air/fuel ratio at a level around the theoretical air/fuel ratio
in order to retain a purifying window of the three-way
catalyst.
[0008] By the way, how to reduce the amount of a noble metal
catalyst used in the aforementioned three-way catalyst is an
important element to be considered from the perspective of cost
competitiveness. However, when the amount of a noble metal catalyst
in a three-way catalyst is significantly reduced, the catalytic
activity also decreases significantly. Thus results in
significantly degraded OSC performance and low-temperature activity
as well as significantly degraded NO.sub.x purification performance
under a high-temperature environment, and the like. This occurs
because a significantly reduced amount of a noble metal catalyst
results in a significantly reduced number of active sites, and a
significantly reduced number of catalytic reaction sites results in
significantly degraded purification performance.
[0009] Among noble metal catalysts, such as Pt, Pd, and Rh, that
are particularly used for a three-way catalyst, Rh is the most
excellent in the NO.sub.x purification performance, but is sold at
the highest market price per unit weight. It has been known that
when Rh is carried on a support that contains cerium oxide (ceria),
higher OSC performance is exhibited. To the contrary, it has been
also known that increasing the amount of cerium oxide in the
support will degrade the NO.sub.x purification performance that is
a characteristic of Rh. Thus, when using Rh as a noble metal
catalyst for a three-way catalyst, producing a three-way catalyst
that is optimal in both the OSC performance and the NO.sub.x
purification performance is an urgent object to be achieved in the
technical field.
[0010] Herein, Patent Document 1 discloses an exhaust gas purifying
catalyst including a first oxygen storage material on which no
noble metal is carried and that has a pyrochlore phase type regular
array structure; and a second oxygen storage material that has a
higher oxygen storage rate and a lower oxygen storage capacity than
the first oxygen storage material, in which a platinum group noble
metal is carried on the second oxygen storage material. According
to such an exhaust gas purifying catalyst, it is possible to
provide an exhaust gas purifying catalyst that has high NO.sub.x
purification performance after endurance.
[0011] Meanwhile, Patent Document 2 discloses an exhaust gas
purifying catalyst having a first catalyst layer and a second
catalyst layer that are sequentially formed on a support substrate,
the first catalyst layer having rhodium carried thereon, and the
second catalyst layer having platinum and palladium carried
thereon, in which the ratio of the carried amount of palladium (y)
to the carried amount of platinum (x) (y/x; molar ratio) satisfies
0<y/x.ltoreq.1.0. According to such an exhaust gas purifying
catalyst, it is possible to provide an exhaust gas purifying
catalyst of a NO.sub.x storage-reduction type that has more
excellent NO.sub.x purification performance.
[0012] By the way, when rhodium is carried on ceria, metallization
of the rhodium is hindered, and the NO.sub.x purification
performance is thus degraded as described above. Meanwhile, when
the amount of ceria is increased to increase the oxygen storage
capacity, a pressure loss is increased.
[0013] That is, even when the exhaust gas purifying catalyst
disclosed in Patent Document 1 or 2 is used, it is unclear whether
or not a catalytic converter that is excellent in both the OSC
performance and the NO.sub.x purification performance can be
provided.
RELATED ART DOCUMENTS
Patent Documents
[0014] Patent Document 1: JP 2012-024701 A
[0015] Patent Document 2: JP 2010-201284 A
SUMMARY
[0016] The present invention has been made in view of the foregoing
problems, and it is an object of the present invention to provide a
catalytic converter with excellent OSC performance and NO.sub.x
purification performance.
[0017] In order to achieve the above object, a catalytic converter
in accordance with the present invention includes a substrate with
a cell structure through which exhaust gas flows, and a catalyst
layer formed on a cell wall surface of the substrate. The catalyst
layer includes a lower catalyst layer and an upper catalyst layer,
the lower catalyst layer being formed on a surface of the
substrate, and the upper catalyst layer being formed on a surface
of the lower catalyst layer. The upper catalyst layer includes at
least a zirconia support with rhodium carried thereon, and two
types of ceria-zirconia-based composite oxides with different
specific surface areas, each of the ceria-zirconia-based composite
oxides having no rhodium carried thereon. The lower catalyst layer
includes an alumina support with platinum carried thereon, and a
ceria-zirconia-based composite oxide.
[0018] The catalytic converter of the present invention is
characterized by having a catalyst layer with a two-layer structure
of a lower catalyst layer, which is formed on the surface of a
substrate, and an upper catalyst layer formed thereon, in which the
upper catalyst layer includes at least a zirconia support with
rhodium carried thereon, and two types of ceria-zirconia-based
composite oxides with different specific surface areas, each of the
ceria-zirconia-based composite oxides having no rhodium carried
thereon. The inventors have verified that as rhodium is carried on
a support that does not contain ceria, the activity of the rhodium
is increased and the NO.sub.x purification rate is thus increased.
In addition, as the upper catalyst layer includes two types of
ceria-zirconia-based composite oxides with different specific
surface areas, more specifically, as the upper catalyst layer
contains a ceria-zirconia-based composite oxide with a large
specific surface area, the NO.sub.x purification rate is increased,
while as the upper catalyst layer also contains a
ceria-zirconia-based composite oxide with a small specific surface
area, an increase in the pressure loss can be suppressed.
[0019] Herein, as the substrate with a cell structure, it is
possible to use, in addition to cordierite made of a composite
oxide of magnesium oxide, aluminum oxide, and silicon dioxide, or
ceramic materials, such as silicon carbide, materials other than
ceramic materials, such as metal materials. In addition, the
substrate may have a so-called honeycomb structure with a number of
cells whose lattice contour is a quadrangle, a hexagon, an octagon,
or the like.
[0020] The lower catalyst layer that is formed on the cell wall
surface of the substrate contains alumina (Al.sub.2O.sub.3), which
is a support, and platinum (Pt) carried thereon, and further
contains a ceria-zirconia-based composite oxide
(CeO.sub.2--ZrO.sub.2 composite oxide).
[0021] Meanwhile, the upper catalyst layer may contain, as
described above, zirconia (ZrO.sub.2), which is a support, and
rhodium (Rh) carried thereon, and also contains two types of
ceria-zirconia-based composite oxides (CeO.sub.2--ZrO.sub.2
composite oxides) with different specific surface areas (SSA), and
may further contain alumina (Al.sub.2O.sub.3).
[0022] Herein, according to a preferred embodiment of the catalytic
converter in accordance with the present invention, of the two
types of ceria-zirconia-based composite oxides with different
specific surface areas, the ceria-zirconia-based composite oxide
with a larger specific surface area has a specific surface area of
greater than or equal to 40 m.sup.2/g, and is contained in the
upper catalyst layer by an amount of greater than or equal to 12
g/L, while the ceria-zirconia-based composite oxide with a smaller
specific surface area has a specific surface area of less than or
equal to 4 m.sup.2/g, and is also contained in the upper catalyst
layer by an amount of greater than or equal to 8 g/L.
[0023] In the present invention, of the two types of
ceria-zirconia-based composite oxides with different specific
surface areas, the "ceria-zirconia-based composite oxide with a
larger specific surface area" has a specific surface area of
greater than or equal to 40 m.sup.2/g, while the
"ceria-zirconia-based composite oxide with a smaller specific
surface area" has a specific surface area of less than or equal to
4 m.sup.2/g.
[0024] The inventors have identified that when each of the
ceria-zirconia-based composite oxide with a larger specific surface
area is contained by an amount of greater than or equal to 12 g/L
in the upper layer and the ceria-zirconia-based composite oxide
with a smaller specific surface area is contained in the upper
catalyst layer by an amount of greater than or equal to 8 g/L, it
is possible to suppress a pressure loss while exhibiting excellent
OSC performance.
[0025] In addition, the zirconia support with rhodium carried
thereon is preferably contained in the upper catalyst layer by an
amount of 40 g/L.
[0026] When the amount of the zirconia support with rhodium carried
thereon is increased, the low-temperature activity is improved, in
particular. However, if the amount of the zirconia support is
increased too much, a pressure loss is increased. Thus, from the
perspective of achieving both excellent low-temperature activity
and suppressed pressure loss, the preferable amount of the zirconia
support with rhodium carried thereon is defined as 40 g/L.
[0027] Further, in another embodiment of the catalytic converter in
accordance with the present invention, the upper catalyst layer is
formed in the range of 80% of the total length of the substrate
from an end of the substrate on the downstream side of the exhaust
gas flow direction, while the lower catalyst layer is formed in the
range of 80% of the total length of the substrate from an end of a
substrate on the upstream side of the exhaust gas flow direction.
It should be noted that each of the upper catalyst layer and the
lower catalyst layer is preferably formed in the range of 65 to 95%
of the total length of the substrate.
[0028] The catalytic converter of the present invention preferably
has a cordierite honeycomb support with excellent resistance to
thermal shock. Alternatively, the catalytic converter may be an
electrically heated converter (EHC). The electrically heated
catalytic converter of this type has a honeycomb catalyst and a
pair of electrodes attached thereto. When current is supplied to
the pair of electrodes to heat the honeycomb catalyst, the activity
of the honeycomb catalyst is increased, so that exhaust gas passing
through the honeycomb catalyst is detoxified. When such a converter
is applied to an exhaust system for exhaust gas that connects a
vehicle engine and a muffler, it is possible to purify exhaust gas
at room temperature and also purify exhaust gas at cold temperature
by activating the catalyst through electric heating.
[0029] As can be understood from the foregoing description, the
catalytic converter of the present invention has a catalyst layer
with a two-layer structure of a lower catalyst layer, which is
formed on the surface of a substrate, and an upper catalyst layer
formed thereon. The upper catalyst layer includes at least a
zirconia support with rhodium carried thereon, and two types of
ceria-zirconia-based composite oxides with different specific
surface areas, each of the ceria-zirconia-based composite oxides
having no rhodium carried thereon. Thus, it is possible to provide
a catalyst converter with excellent OSC performance and NO.sub.x
purification performance while at the same time suppressing a
pressure loss.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a schematic view of a catalytic converter of the
present invention.
[0031] FIG. 2 is a partially enlarged view of a cell.
[0032] FIG. 3 is a longitudinal sectional view illustrating an
embodiment of a catalyst layer.
[0033] FIG. 4 is a graph showing the experimental results for
identifying the relationship between the Ce concentration in the Rh
support and the NO.sub.x purification rate.
[0034] FIG. 5 is a graph showing the experimental results for
identifying the relationship between the added amounts of a high
SSA composite oxide and a low SSA composite oxide in the upper
catalyst layer and the NO.sub.x purification rate.
[0035] FIG. 6 is a graph showing the experimental results for
identifying the relationship between the amount of the Rh support
and a pressure loss and the relationship between the amount of the
Rh support and the purification temperature of low-temperature
active NO.sub.x.
DETAILED DESCRIPTION OF THE EMBODIMENT(S)
[0036] Hereinafter, embodiments of a catalytic converter of the
present invention will be described with reference to the drawings.
The catalytic converter shown in the drawings has an upper catalyst
layer that is formed in the range of 80% of the total length of a
substrate from an end of the substrate on the downstream side of
the exhaust gas flow direction, and also has a lower catalyst layer
that is formed in the range of 80% of the total length of the
substrate from an end of the substrate on the upstream side of the
exhaust gas flow direction. It should be noted that the length over
which each of the upper catalyst layer and the lower catalyst layer
is formed is preferably in the range of 65 to 95% of the total
length of the substrate.
(Exhaust System for Exhaust Gas)
[0037] First, an exhaust system for exhaust gas in which the
catalytic converter of the present invention is provided will be
briefly described. An exhaust system for exhaust gas to which the
catalytic converter of the present invention is applied has a
configuration in which an engine, a catalytic converter, a
three-way catalytic converter, a sub-muffler, and a main muffler
are arranged and are mutually connected with system pipes so that
exhaust gas generated in the engine flows through each part via the
system pipe and is then discharged. Next, an embodiment of the
catalytic converter will be described.
(Embodiment of Catalytic Converter)
[0038] FIG. 1 is a schematic view of the catalytic converter of the
present invention. FIG. 2 is a partially enlarged view of a cell.
FIG. 3 is a longitudinal sectional view illustrating an embodiment
of a catalyst layer.
[0039] A catalytic converter 10 shown in FIG. 1 generally includes
a cylindrical substrate 1 with a number of cells and a catalyst
layer 3 formed on the surface of a cell wall 2 of each cell as
shown in FIG. 2.
[0040] Herein, examples of the substrate 1 include cordierite made
of a composite oxide of magnesium oxide, aluminum oxide, and
silicon dioxide, ceramic materials, such as silicon carbide, and
materials other than ceramic materials, such as metal
materials.
[0041] The substrate 1 has a honeycomb structure with a number of
cells whose lattice contour is a quadrangle, a hexagon, an octagon,
or the like. Exhaust gas, which has entered a cell at an end of the
substrate 1 on the upstream side (Fr side) of the exhaust gas flow
direction, flows through the substrate 1, and is purified in this
process, and then, the purified exhaust gas flows out from an end
of the substrate 1 on the downstream side (Rr side) of the exhaust
gas flow direction (x-direction).
[0042] Next, an embodiment of the catalyst layer will be described
with reference to FIGS. 2 and 3.
[0043] The catalyst layer 3 shown in FIGS. 2 and 3 includes a lower
catalyst layer 4 that is formed on the surface of a cell wall 2 and
an upper catalyst layer 5 that is formed on the surface of the
lower catalyst layer 4.
[0044] The lower catalyst layer 4 is formed in the range of 80% of
the total length of the substrate 1 from the end of the substrate 1
on the upstream side Fr of the exhaust gas flow direction, while
the upper catalyst layer 5 is formed in the range of 80% of the
total length of the substrate 1 from the end of the substrate 1 on
the downstream side Rr of the exhaust gas flow direction.
[0045] Herein, the lower catalyst layer 4 contains an alumina
support (Al.sub.2O.sub.3) with platinum (Pt) carried thereon, and a
ceria-zirconia-based composite oxide (CeO.sub.2--ZrO.sub.2
composite oxide).
[0046] Meanwhile, the upper catalyst layer 5 contains a zirconia
support (ZrO.sub.2) with rhodium (Rh) carried thereon, and two
types of ceria-zirconia-based composite oxides
(CeO.sub.2--ZrO.sub.2 composite oxides) with different specific
surface areas, and further contains alumina (Al.sub.2O.sub.3).
[0047] Regarding the two types of ceria-zirconia-based composite
oxides with different specific surface areas, the specific surface
area of the ceria-zirconia-based composite oxide with a larger
specific surface area is greater than or equal to 40 m.sup.2/g,
while the specific surface area of the ceria-zirconia-based
composite oxide with a smaller specific surface area is less than
or equal to 4 m.sup.2/g.
[0048] In the upper catalyst layer 5, rhodium (Rh) is carried only
on zirconia (ZrO.sub.2) that does not contain ceria. Such a
structure can improve the NO.sub.x purification rate. This has been
verified through experiments described below.
[0049] As the upper catalyst layer 5 contains a
ceria-zirconia-based composite oxide with a large specific surface
area and a ceria-zirconia-based composite oxide with a small
specific surface area, more specifically, as the upper catalyst
layer 5 contains a composite oxide with a large specific surface
area by an amount of greater than or equal to 12 g/L and also
contains a composite oxide with a small specific surface area by an
amount of greater than or equal to 8 g/L, it is possible to
suppress a pressure loss while exhibiting excellent OSC
performance. This has been also verified through experiments
described below.
[0050] In addition, the zirconia support with rhodium carried
thereon is contained in the upper catalyst layer 5 by an amount of
40 g/L.
[0051] When the amount of the zirconia support with rhodium carried
thereon is increased, the low-temperature activity, in particular,
is improved. However, if the amount of the zirconia support is
increased too much, a pressure loss is increased. Thus, from the
perspective of achieving both excellent low-temperature activity
and suppressed pressure loss, the amount of the zirconia support
with rhodium carried thereon is defined as 40 g/L. This has been
also verified through experiments described below.
[0052] (Experiments for identifying the relationship between the Ce
concentration in the Rh support and the NO.sub.x purification rate,
experiments for identifying the relationship between the added
amounts of a high SSA composite oxide and a low SSA composite oxide
in the upper catalyst layer and the NO.sub.x purification rate, and
experiments for identifying the relationship between the amount of
the Rh support and a pressure loss and the relationship between the
amount of the Rh support and the purification temperature of
low-temperature active NO.sub.x)
[0053] The inventors have conducted experiments for identifying the
relationship between the Ce concentration in the Rh layer and the
NO.sub.x purification rate, experiments for identifying the
relationship between the added amounts of a high SSA composite
oxide and a low SSA composite oxide in the upper catalyst layer and
the NO.sub.x purification rate, and experiments for identifying the
relationship between the amount of the Rh support and a pressure
loss and the relationship between the amount of the Rh support and
the purification temperature of low-temperature active NO.sub.x.
Reference Examples 1-12, Example 1, and Comparative Examples 1-4
were produced using methods described below.
REFERENCE EXAMPLE 1
[0054] In Reference Example 1, the lower catalyst layer contains Pt
as a catalyst (Pt(0.2)/Al.sub.2O.sub.3(25)+CZ(30)), and the upper
catalyst layer contains Rh as a catalyst (Rh(0.12)/Ce--Zr composite
oxide A(40)+Al.sub.2O.sub.3(20)). Herein, the unit of the numerical
values in the parentheses is g/L. First, using nitric acid Pt,
Pt/Al.sub.2O.sub.3 (i.e., material 1) in which Pt is carried on
Al.sub.2O.sub.3 was prepared. Impregnation was used as a method for
causing Pt to be carried on Al.sub.2O.sub.3. Next, a slurry 1 was
prepared by pouring the material 1, a CZ material, and a
Al.sub.2O.sub.3-based binder into distilled water while agitating
them. Further, the prepared slurry 1 was poured into a substrate,
and unnecessary portions were wiped away with a blower, so that the
wall surface of the substrate was coated with the slurry 1. At that
time, the coating material for the Pt layer was prepared such that
the content of Pt, the content of the material 1, and the content
of the CZ material with respect to the volume of the substrate were
0.2 g/L, 25 g/L, and 30 g/L, respectively. Finally, moisture was
dried with a dryer kept at 120.degree. C. for two hours, and baking
was performed with an electric furnace at 500.degree. C. for 2
hours. Likewise, using nitric acid Rh, a Rh/Ce--Zr composite oxide
A (i.e., material 2) in which Rh is carried on a Ce--Zr composite
oxide A was prepared. Herein, the proportion of Ce in the Ce--Zr
composite oxide A was 30 mass %. Next, a slurry 2 was prepared by
pouring the material 2, Al.sub.2O.sub.3, and an
Al.sub.2O.sub.3-based binder into distilled water while agitating
them such that the materials were suspended in the distilled water.
The prepared slurry 2 was poured into the coated substrate, and
unnecessary portions were wiped away with a blower, so that the
wall surface of the substrate was coated with the slurry 2. At that
time, the coating material for the Rh layer was prepared such that
the content of Rh, the content of the material 2, and the content
of Al.sub.2O.sub.3 with respect to the volume of the substrate were
0.12 g/L, 40 g/L, and 20 g/L, respectively. Finally, moisture was
dried with a dryer kept at 120.degree. C. for two hours, and baking
was performed with an electric furnace at 500.degree. C. for 2
hours.
REFERENCE EXAMPLES 2, 3, AND 4
[0055] In each of Reference Examples 2, 3, and 4, the lower
catalyst layer contains Pt as a catalyst
(Pt(0.2)/Al.sub.2O.sub.3(25)+CZ(30)), and the upper catalyst layer
contains Rh as a catalyst (Rh(0.12)/Ce--Zr composite oxide
B,C,D(40)+Al.sub.2O.sub.3(20)). A slurry was prepared by changing
the specifications of the Rh support (material 2) used for the
slurry 2 in Reference Example 1, and then, coating, drying, and
baking were performed. With respect to the catalyst, the process
was unchanged except that the composition of the material 2 shown
in Reference Example 1 was changed. In Reference Example 2, a
Ce--Zr composite oxide that contains a Ce concentration of 0 mass %
(B) was used; in Reference Example 3, a Ce--Zr composite oxide that
contains a Ce concentration of 20 mass % (C) was used; and in
Reference Example 4, a Ce--Zr composite oxide that contains a Ce
concentration of 60 mass % (D) was used.
REFERENCE EXAMPLES 5, 6, 7, AND 8
[0056] In each of Reference Examples 5, 6, 7, and 8, the lower
catalyst layer contains Pt as a catalyst
(Pt(0.2)/Al.sub.2O.sub.3(25)+CZ(30)), and the upper catalyst layer
contains Rh as a catalyst (Rh layer Rh(0.12)/Ce--Zr composite oxide
B(40)+Al.sub.2O.sub.3(20)+low SSA Ce--Zr composite
oxide(4,8,12,16)). A slurry was prepared by further pouring each
amount of a low SSA Ce--Zr composite oxide into the slurry 2 in
Reference Example 1, and then, coating, drying, and baking were
performed. With respect to the catalyst, the process was unchanged
except that a low SSA Ce--Zr composite oxide was added in the step
of preparing the slurry 2 in Reference Example 1. In Reference
Example 5, 4 parts by mass of a low SSA Ce--Zr composite oxide was
added; in Reference Example 6, 8 parts by mass of a low SSA Ce--Zr
composite oxide was added; in Reference Example 7, 12 parts by mass
of a low SSA Ce--Zr composite oxide was added; and in Reference
Example 8, 16 parts by mass of a low SSA Ce--Zr composite oxide was
added. It should be noted that the "low SSA Ce--Zr composite oxide"
herein means a material that initially has an SSA of less than or
equal to 4 m.sup.2/g.
REFERENCE EXAMPLES 9, 10, 11, AND 12
[0057] In each of Reference Examples 9, 10, 11, and 12, the lower
catalyst layer contains Pt as a catalyst
(Pt(0.2)/Al.sub.2O.sub.3(25)+CZ(30)), and the upper catalyst layer
contains Rh as a catalyst (Rh(0.12)/Ce--Zr composite oxide
B(40)+Al.sub.2O.sub.3(20)+high SSA Ce--Zr composite oxide
(4,8,12,16)). A slurry 2 was prepared by further pouring each
amount of a high SSA Ce--Zr composite oxide into the slurry 2 in
Reference Example 1, and then, coating, drying, and baking were
performed. With respect to the catalyst, the process was unchanged
except that a high SSA Ce--Zr composite oxide was added in the step
of preparing the slurry 2 in Reference Example 1. In Reference
Example 9, 4 parts by mass of a high SSA Ce--Zr composite oxide was
added; in Reference Example 10, 8 parts by mass of a high SSA
Ce--Zr composite oxide was added; in Reference Example 11, 12 parts
by mass of a high SSA Ce--Zr composite oxide was added; and in
Reference Example 12, 16 parts by mass of a high SSA Ce-Zr
composite oxide was added. It should be noted that the "high SSA
Ce--Zr composite oxide" herein means a material that initially has
an SSA of greater than or equal to 40 m.sup.2/g.
EXAMPLE 1 AND COMPARATIVE EXAMPLES 1, 2, 3, AND 4
[0058] In each of Example 1 and Comparative Examples 1, 2, 3, and
4, the lower catalyst layer contains Pt as a catalyst
(Pt(0.2)/Al.sub.2O.sub.3(25)+CZ(30)), and the upper catalyst layer
contains Rh as a catalyst (Rh(0.12)/Ce--Zr composite oxide
B(x)+Al.sub.2O.sub.3(20)+low SSA Ce--Zr composite oxide(8)+high SSA
Ce--Zr composite oxide (12)). A slurry was prepared by changing the
Rh support of the slurry 2 in Reference Example 1 to a Ce--Zr
composite oxide B that contains 0 mass % C and pouring 8 parts by
mass of a low SSA Ce--Zr composite oxide and 12 parts by mass of a
high SSA Ce--Zr composite oxide into the slurry 2, and then,
coating, drying, and baking were performed. With respect to the
catalyst, the process was unchanged except that the material for
the step of preparing the slurry 2 in Reference Example 1 was
changed. In Comparative Example 1, 24 parts by mass of a Ce--Zr
composite oxide B was added; in Comparative Example 2, 32 parts by
mass of a Ce--Zr composite oxide B was added; in Example 1, 40
parts by mass of a Ce--Zr composite oxide B was added; in
Comparative Example 3, 48 parts by mass of a Ce--Zr composite oxide
B was added; and in Comparative Example 4, 56 parts by mass of a
Ce--Zr composite oxide B was added.
<Evaluation Method>
[0059] A 4.3 L V8 cylinder gasoline engine was used, and the bed
temperature of a catalyst on the downstream side was set to
950.degree. C., so that a cycle that includes feedback, fuel cut,
rich, and lean per minute as a condition was conducted for 50
hours.
[0060] To measure the NO purification rate under fluctuating
conditions of the A/F ratio, a catalyst layer on the upstream side
that an aged catalytic converter was mounted, and the purification
rate for when the entering gas atmosphere was periodically switched
between the rich and lean sides of the A/F ratio was measured. In
addition, to measure the NO purification rate under the steady rich
condition, an aged catalytic converter was mounted, and the
purification rate for when the entering gas atmosphere was
continuously maintained on the rich side of the A/F ratio was
measured. Further, to measure a pressure loss, a pressure loss for
when air was supplied to a catalyst at a constant flow rate (6
m.sup.3/minute) was measured. Furthermore, regarding NO light-off
performance NO.sub.x-T50, the temperature that had increased from a
low temperature under the rich environment and at which the NO
purification rate reached 50% was measured.
[0061] Table 1 below shows the materials used herein.
TABLE-US-00001 TABLE 1 Portion Name of Material Producer
Composition Upper Ce--Zr TOYOTA MOTOR CeO.sub.2 (x mass %),
Catalyst Composite CORPORATION ZrO.sub.2 (100-x mass %) Oxide Layer
High, TOYOTA MOTOR Al.sub.2O.sub.3 (30 mass %), (Rh Low SSA
CORPORATION CeO.sub.2 (20 mass %), Layer) Ce--Zr ZrO.sub.2 (40 mass
%), Composite Y.sub.2O.sub.3 (4 mass %), Oxide Nd.sub.2O.sub.3 (2
mass %), La.sub.2O.sub.3 (4 mass %) Alumina Sasol Al.sub.2O.sub.3
(99 mass %), La.sub.2O.sub.3 (1 mass %) Lower CZ Material Rhodia
CeO.sub.2 (30 mass %), Catalyst ZrO.sub.2 (60 mass %),
La.sub.2O.sub.3 (5 mass %), Y.sub.2O.sub.3 (5 mass %) Layer Alumina
Sasol Al.sub.2O.sub.3 (99 mass %), (Pt La.sub.2O.sub.3 (1 mass %)
Layer)
<Results of Experiment>
[0062] FIGS. 4 to 6 each show the experimental results. Herein,
FIG. 4 is a graph showing the experimental results for identifying
the Ce concentration in the Rh support and the NO.sub.x
purification rate. In addition, FIG. 5 is a graph showing the
experimental results for identifying the relationship between the
added amounts of a high SSA composite oxide and a low SSA composite
oxide in the upper catalyst layer and the NO.sub.x purification
rate. Further, FIG. 6 is a graph showing the experimental results
for identifying the relationship between the amount of the Rh
support and a pressure loss and the relationship between the amount
of the Rh support and the NO.sub.x light-off performance.
[0063] FIG. 4 can confirm that regarding the Ce concentration in
the Rh support, the catalytic activity becomes the highest when the
concentration is 0%. Accordingly, a Ce concentration of 0% in
Reference Example 2 is adopted. It is considered that the activity
of Rh is improved with a reduction in the amount of bases in the Rh
support.
[0064] In addition, FIG. 5 can confirm that the OSC performance of
a low SSA-OSC material is saturated when it is added by an amount
of greater than or equal to 8 g/L, and the OSC performance of a
high SSA-OSC material is also saturated when it is added by an
amount of greater than or equal to 12 g/L from the difference from
the required OSC performance, and thus that the low SSA-OSC
material is preferably added by an amount of greater than or equal
to 8 g/L and the high SSA-OSC material is preferably added by an
amount of greater than or equal to 12 g/L. It should be noted that
the low SSA-OSC material is useful for reducing a pressure loss as
it has a small volume relative to its weight. However, the low
SSA-OSC material has a limitation in the oxygen release capacity of
the OSC performance. Thus, compensating for the OSC performance
that is insufficient with the low SSA-OSC by using a high-SSA OSC
material can achieve both excellent OSC performance and a reduction
in the pressure loss.
[0065] In addition, FIG. 6 can confirm that when the amount of the
Rh support is increased, the light-off performance is improved, but
a pressure loss is increased.
[0066] Increasing the amount of the Rh support to increase the
dispersibility of Rh is useful, and the light-off performance will
be improved with an increase in the amount of the Rh support.
Meanwhile, as a pressure loss tends to increase, a Rh support with
an amount of 40 g/L, which is shown by Example 1, can be defined as
the optimal amount of the Rh support.
[0067] It should be noted that the results shown in FIG. 6 indicate
not a point that Comparative Examples 1-4 should be excluded from
the scope of the present invention, but a point that Example 1 has
an optimal amount of the Rh support that can achieve both excellent
OSC performance and a reduction in the pressure loss.
[0068] Although the embodiments of the present invention have been
described in detail with reference to the drawings, specific
structures are not limited thereto, and any design changes that may
occur within the spirit and scope of the present invention are all
included in the present invention.
DESCRIPTION OF SYMBOLS
[0069] 1 Substrate [0070] 2 Cell Wall [0071] 3 Catalyst Layer
[0072] 4 Lower Catalyst Layer [0073] 5 Upper Catalyst Layer [0074]
10 Catalytic converter [0075] Fr Upstream side of the exhaust gas
flow direction [0076] Rr Downstream side of the exhaust gas flow
direction
* * * * *